/Engine/Source/Runtime/AIModule/Classes/EnvironmentQuery/Tests/EnvQueryTest_GameplayTags.h
Calling function only makes sense before first run of given query by the EQS manager. The query gets preprocessed and cached then so the query value will get stored and calling this function will not change it (unless you call it on the cached test's instance, seeUEnvQueryManager::CreateQueryInstance).
void SetTagQueryToMatch ( [FGameplayTagQuery](API\Runtime\GameplayTags\FGameplayTagQuery) & GameplayTagQuery )